Clinical quantitative flow cytometry is a new technology that utilizes specialized antibody staining techniques and fluorescent standards to quantitate antibody binding to different cell populations within a sample. The Flow Cytometry Unit uses this technique to quantitate tumor cell antibody binding capacity in patients undergoing various antibody based therapies. The flow cytometric assays are performed in place of previous studies using radioactively labeled antibodies. The flow cytometric assays are more rapid, make the use of radioactivity unnecessary and are performed on 100 &#61549;L of blood instead of 25mL. The data collected is also more precise and improves the ability of NCI investigators to compare antibody binding to tumor cells to treatment response. Quantitation of fluorescent antibody binding immediately post therapy also allows precise determination of saturation of antigen sites with non labeled therapeutic antibody. As the majority of NCI lymphoma and leukemia protocols include antibody based therapy in their regimens, this is a tremendous resource for NCI investigators. Antibody binding quantitation is not provided by reference laboratories. Antibody binding quantitation is currently being provided for NCI patients receiving the following therapeutic antibodies under NCI protocols: anti-CD20 (Rituximab); anti-CD22 (BL-22); anti-CD25 (anti-Tac for T cell and LMB-2 for B cell malignancies); anti-CD52 (Campath); and CD2 (MEDI507). The Flow Cytometry Unit measures apoptosis in malignant and normal cells post exposure to therapeutic agents in vivo and in vitro. New treatment modalities are being developed to attempt to induce apoptosis in malignant cells while leaving the normal cells unaffected. Flow cytometry is ideally suited for measuring apoptosis in specimens containing mixed populations. Using multiple antibodies to detect the tumor cells among reactive, non-malignant cells, one can separately measure apoptosis in the tumor cells and the normal cells. This helps to determine the efficacy of experimental therapies in different diseases and the effects on normal cells. The Flow Cytometry unit is analyzing patient specimens collected pre and post therapy and patient cells that have been incubated in vitro with new potential therapeutic agents to determine efficacy prior to administration to patients.